In many processes, for example, in the area of process engineering, chemistry or mechanical engineering, gas concentrations must be reliably determined and/or a defined quantity of a gas mass, in particular an air mass, must be supplied. These include in particular combustion processes, which must proceed under controlled conditions. An important example is the combustion of fuel in internal combustion engines of motor vehicles, in particular those having subsequent catalytic exhaust gas purification. Another area of application is the supply of gases of a highly specific composition for fuel cells. Safety-relevant applications are also of significance. For example, a hydrogen sensor may be used in fuel cell vehicles in order to warn passengers of a slow escape of hydrogen. Air becomes ignitable at a hydrogen concentration of roughly 4% and even becomes explosive at higher concentrations so that the hydrogen sensor may be linked, for example, to an appropriate warning device or an appropriate automatic emergency system. Other safety-relevant applications of gas sensors of this type are also conceivable.
Various types of sensors are used for measuring a gas stream and/or a gas concentration. One category of such sensors includes sensors having a sensor chip. A type of sensor of this category known from the related art is the hot film air mass sensor (HFM), an embodiment of which is described, for example, in German Patent Application No. DE 196 01 791. Normally used in such hot film air mass sensors is a sensor chip having a thin sensor diaphragm, for example a silicon sensor chip. Typically, at least one heating resistor is situated on the sensor diaphragm, the heating resistor being surrounded by one or more temperature measuring resistors (temperature sensors). The temperature distribution in an air stream passing across the diaphragm changes, which may in turn be detected by the temperature measuring resistors and analyzed using an actuation and analysis circuit. It is thus possible, for example, to determine an air mass flow from a resistance difference of the temperature measuring resistors. Various other versions of this sensor type are known from the related art.
In addition to the detection of a flow, the detection and measurement of components making up the particular gaseous fluid is also of great significance. A sensing principle is based on the varying thermal capacity and/or thermal conductivity of the different fluid components and is described, for example, in M. Arndt: “Micromachined Thermal Conductivity Hydrogen Detector for Automotive Applications,” Sensors, 2002. Proceedings of IEEE. For example, the detection of hydrogen in an air-hydrogen mixture makes use of the fact that hydrogen has a higher thermal conductivity than air, i.e., the components of air. In a sensor configuration designed similar to hot film air mass sensors (HFM), an air-hydrogen mixture diffuses through a thin diaphragm or a tight mesh into a measuring space of a sensor. The presence of hydrogen in the gaseous fluid changes the temperature of the heated measuring diaphragm or its thermal output which is given off to the ambient air. A measuring signal is generated from this which reflects the concentration of the hydrogen.
As described above, typical chip gas sensors are designed in such a way that they have a sensor diaphragm (for example, a silicon diaphragm) having low thermal conductivity and a surrounding chip body. Electrically conductive structures are situated on this sensor diaphragm. However, the chip frequently cracks and/or fractures when such sensors are put into practical use, in particular in the area of the transition between the sensor diaphragm and the chip body, usually due to the presence there of thermal and/or mechanical stresses related to design or operation. Experience shows that these fractures or cracks mostly extend in or along the edges of the sensor diaphragm at the transition to the chip body. Such fractures may result in complete or partial failure of the sensor and/or the output of corrupted signals. As long as these cracks or fractures do not affect any printed conductors of the sensor, the sensor will usually continue to generate electrical signals; however, they are corrupted due, for example, to the changed thermal conductivity of the sensor diaphragm and/or the changed thermal connection of the diaphragm to the chip body. Because the sensor is used in safety-relevant applications in many cases, for example, in fuel cells, such erroneous indications are usually not tolerable.